Measure from the frame to ground on a level surface, then start measuring at varies points side to side till you find the problem. If you can't find the problem, level the truck on jack stands where it looks right, then start measuring again to see what has changed. If you can't find a level surface, borrow someones trailer and put the truck on the trailer. Then level the trailer front to back and side to side using jacks and blocks.

You could have poorly assembled front sheet metal
wrong front fenders
bent frame
twisted frame
bent front axle
weak rear spring
wrong length shackle
weak or missing cab mounts
rotted out cab mounts
wrong spring mount

In 70 years of service, any number of problems can cause the truck to lean.

1937 Chevrolet Inline-powered Streetrod